Many electronic and computer-related products such as semiconductors, hard disks and CD-ROMS require highly polished or planarized surfaces in order to achieve optimum performance. In the semiconductor manufacturing industry, for example, silicon workpieces are used in the manufacture of integrated circuit components and the like. The workpieces are known in the industry as "wafers" and typically have a flat, circular disk-like shape. The wafers are initially sliced from a silicon ingot and, thereafter, undergo multiple masking, etching, and dielectric and conductor deposition processes to create microelectronic structures and circuitry on the wafers. The surface of a wafer undergoing these processes typically must be polished or planarized between processing steps to ensure proper flatness, permitting use of photolithographic processes for building additional dielectric and metallization layers on the wafer surface.
Chemical Mechanical Planarization (CMP) machines have been developed to polish or planarize silicon wafer surfaces to the flat condition necessary for manufacture of integrated circuit components and the like. CMP processes and machines are known in the art and are described in several U.S. Patents. Examples include U.S. Pat. No. 4,805,348, issued in February 1989, to Arai, et al.; U.S. Pat. No. 4,811,522, issued in March 1989 to Gill; U.S. Pat. No. 5,099,614, issued in March, 1992 to Arai et al.; U.S. Pat. No. 5,329,732, issued in July, 1994 to Karlsrud et al.; U.S. Pat. No. 5,476,414, issued in December 1995 to Masayoshi et al.; U.S. Pat. Nos. 5,498,196 and 5,498,199, both issued in March, 1996 to Karlsrud et al.; and U.S. Pat. No. 5,558,568, issued in September 1996 to Talieh et al.
Known CMP machines and processes typically include loading and unloading stations where wafers are loaded into and unloaded from wafer transport cassettes, as well as a polishing station or table where the wafers are polished. A transition station is ordinarily positioned between the load/unload stations and the polishing station to temporarily accommodate wafers as they are transferred between processing stations. The wafers are usually moved between the load/unload stations and the transition station through the use of precision robotics and transfer flippers, and are transported between the transition station and polishing station through the use of a multiple head wafer transport apparatus.
In conventional CMP machines, the transition station takes the form of a rotatable index table having a plurality of alternating receive and discharge stations for holding wafers. The index table rotates or indexes as necessary to position wafers for transfer to and from the load/unload stations as well as for transfer to and from the polishing station. The wafers may be submerged in deionized water or another similar processing solution while they are present on the index table.
Prior art index tables of this sort, while adequate in most respects, do have several drawbacks. Rotation of prior art index tables is accomplished through use of a motor-driven indexing shaft which extends through an opening formed in the center of the table. The indexing shaft engages gear teeth or bolt patterns formed around the inside diameter of the central opening such that rotation of the shaft effects rotation of the table. In conventional CMP machines, however, the central opening of the table has a small diameter (in the range of five inches) which creates difficulty since other tubing, mechanisms, support structures, drains and so forth must also be passed through the opening. The tight fit of the components passed through the opening can lead to undesirable and destructive chaffing or rubbing between the components and the rotating table. Moreover, if the CMP machine includes a secondary polishing table, the space limitations described above, as well as other control limitations, dictate that the secondary table be driven by belt coupling or otherwise mechanically linking it to the main polishing table drive mechanism. Consequently, there is no independent speed control for the two tables. This is a significant drawback, as it is sometimes desirable to rotate the tables at different speeds simultaneously. One example is where it is desired to polish wafers with the secondary table while simultaneously applying conditioner to the polishing pad of the main table.